This seems like a poor argument. I don't like much either having the obligation to give GitHub my phone number, but it's not the same thing as a social security number, now is it ? Would you argue otherwise ?
Github seems to have no legit need for a user's phone number. Since there's not even a way to tell them to go pound sand, I'd say opting out of disclosing sensitive information they don't need by not signing in/up and equating their unreasonable demand with a ban is respectable.